It is well known for lubricating oils to contain a number of surface active additives (including antiwear agents, dispersants, or detergents) used to protect internal combustion engines from wear, soot deposits and acid build up. Often, such surface active additives can have harmful effects on bearing corrosion or fuel economy. A common antiwear additive for engine lubricating oils is zinc dialkyldithiophosphate (ZDDP). It is believed that ZDDP antiwear additives protect the engine by forming a protective film on metal surfaces. ZDDP may have a detrimental impact on fuel economy and efficiency and copper corrosion. Consequently, engine lubricants may also contain a friction modifier to obviate the detrimental impact of ZDDP on fuel economy and corrosion inhibitors to obviate the detrimental impact of ZDDP on copper corrosion.
Further, engine lubricants containing phosphorus compounds and sulphur have been shown to contribute in part to particulate emissions and emissions of other pollutants. In addition, sulphur and phosphorus tend to poison the catalysts used in catalytic converters, resulting in a reduction in performance of said catalysts.
With increasing control of both sulphated ash and release of emissions (typically to reduce NOx formation, SOx formation) there is a desire towards reduced amounts of sulphur, phosphorus and sulphated ash in engine oils. Consequently, the amounts of phosphorus-containing antiwear agents such as ZDDP, overbased detergents such as calcium or magnesium sulphonates and phenates have been reduced. As a consequence, ashless additives such as esters of polyhydric alcohols or hydroxyl containing acids including glycerol monooleate and alkoxylated amines have been contemplated to provide friction performance. However there have been observations that ashless friction modifiers may in some instances increase corrosion of metal, namely, copper or lead. Copper and lead corrosion may be from bearings and other metal engine components derived from alloys using copper or lead. Consequently, there is a need to reduce the amount of corrosion caused by ashless additives. However, reducing the levels of antiwear and other ash-containing additives may result in increasing amounts of wear and/or copper corrosion.
Various attempts have been made to reduce corrosion caused by ashless additives. These attempts include those disclosed in US Patent Application US 2004/038835; U.S. Pat. Nos. 3,966,623, 3,896,050, U.S. Pat. No. 4,012,408; and European publication EP 1 642 954.
US Patent Application US 2004/038835 discloses certain 1,2,4-triazole metal deactivators are especially non-aggressive towards lead engine parts such as bearings. The inclusion of certain 1,2,4-triazole compounds allows the co-use of corrosive additives such as sulfur-containing additives and vegetable oil-derived friction modifiers.
U.S. Pat. No. 3,966,623 discloses improved copper corrosion properties by employing a lubricant that contains a combination of an alkenyl or alkyl primary amine derivative salt of 2-mercaptobenzothiazole, and 2,5-bis-hydrocarbyldithio-1,3,4-thiadiazole. This combination is suitable for reduction of copper corrosion caused by additives with detergent, dispersancy, load carrying and lubricity functions These additives may be corrosive in themselves and/or break down during use into corrosive substances which result in severe corrosive attack.
U.S. Pat. Nos. 3,896,050 and 4,012,408 disclose the problem of copper corrosion caused by lubricant additives. The solution proposed in U.S. Pat. No. 3,896,050 is the use of a 5-bis(alkyldithio)-4-substituted isothiazole as a copper corrosion inhibitor.
EP 1 642 954 discloses a fluid composition comprising at least one hydroxy-substituted carboxylic acid. The at least one hydroxy-substituted carboxylic acid provides at least one property chosen from rust inhibition, corrosion inhibition, improved lubricity, and improved lead compatibility.